Prosthetic shock absorber

ABSTRACT

A prosthesis is provided having a first plate and a second plate spaced apart from the first plate. A substantially round post separates the first plate and the second plate and a flexure is provided in the second plate proximate the post.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is based on and claims the benefit of U.S.provisional patent application having Ser. No. 60/526,728, filed Dec. 3,2003, and is also a continuation-in-part of U.S. patent application Ser.Nos. 10/431,206 and 10/431,207, both filed May 7, 2003, and both nowabandoned, each of which is a continuation-in-part of U.S. patentapplication Ser. No. 09/972,074, filed Oct. 5, 2001 now abandoned; thecontents of the aforementioned applications are hereby incorporated byreference in their entirety.

BACKGROUND OF THE INVENTION

The present invention pertains generally to a joint prosthesis. Inparticular, the present invention relates to absorbing forces in a jointprosthesis.

The human body includes several joints such as the knee, the hip, theshoulder and the elbow. These joints are vulnerable to damage fromvarious injuries, wear and disease. When the joints have been severelydamaged, partial or total joint replacement may be the only viablesolution. In many joint replacements, a prosthetic structure is insertedinto the joint. Typically, the prosthetics include a base member securedto a bone to allow normal joint articulation.

The human knee is the single largest joint of the human body, but due toits structure, is arguably the most vulnerable to damage. The legconsists principally of a lower bone called a tibia and an upper boneknown as the femur. The tibia and femur are hinged together at the kneejoint. The knee joint includes several femoral condyles supported in anengagement with crescentic fibrocartilages that are positioned on theupper end of the tibia and receive the femur. The joint is held togetherby numerous ligaments, muscles and tendons. The patella is a similarlysupported bone positioned in front of the knee joint and acts as ashield for it.

In addition to providing mobility, the knee plays a major role insupporting the body during static and dynamic activities. The knee worksin conjunction with the hip and ankle to support the body weight duringstatic erect posture. The knee is also heavily loaded because of itslocation connecting the two longest bones in the human body. Bodyweight, inertia and ground reaction forces often produce large momentsat the knee. Dynamically, the knee joint must transmit extremely highforces needed for powerful movement of the lower extremity, whiledamping out impulsive shock loads to the spine and head. Furthermore,the knee must provide major stability to the lower extremity as well asfulfill major mobility roles during movement.

In current knee replacement prosthetic designs, the tibia is resected toform a flat, horizontal platform known as a tibial plateau. A tibialplatform is secured to the tibial plateau with posts or anchors fixednormal or perpendicular to the tibia plateau. The anchors provideadditional support to the tibial platform when the joint is subjected toshear, tipping and torque forces present under normal knee articulation.

A similar component, comprising a curved convex semi-spherical shell,covers the femoral condyles and slidably engages a concave tibialbearing insert. On a side opposite the femoral component, the tibialinsert is substantially flat and slidably engages the tibial platform.Interaction of opposing surfaces of these three elements, the femoralcomponent, the tibial component, the tibial insert and the tibialplatform allows the prostheses to function in a manner equivalent to anatural knee joint.

Current prosthetic designs are relatively inflexible and inelastic,especially when reacting to forces produced on the knee joint. When aprosthesis is placed in-vivo, the prosthesis experiences a larger numberof force cycles that can ultimately lead to failure of the prosthesis.As a result, a prosthesis is needed that can absorb and limit failureover a larger number of force cycles.

SUMMARY OF THE INVENTION

A prosthesis is provided having a first plate and a second plate spacedapart from the first plate. A substantially round post connects thefirst plate and the second plate. The second plate includes a flexurepositioned proximate the post to deflect with forces transmitted throughthe post.

In another embodiment, a prosthesis includes a first plate and a secondplate spaced apart from the first plate to form a gap therebetween. Thesecond plate can move independently of the first plate. The prosthesisalso includes a lower plate spaced apart from the first plate and thesecond plate. A plurality of support posts support the first plate andthe second plate above the lower plate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front view of a knee prosthesis according to the presentinvention.

FIG. 2 is an isometric perspective view of a tibia and a tibialcomponent.

FIG. 3 is a front view of a tibial component.

FIG. 4 is a side view of a tibial component.

FIG. 5 is a top perspective view of a medial plateau and a lateralplateau.

FIG. 6 is a bottom perspective view of a medial plateau and a lateralplateau.

FIG. 7 is a top perspective view of a lower plate of a body portion.

FIG. 8 is a bottom perspective view of a lower plate of a body portion.

FIG. 9 is a bottom perspective view of a cover plate.

FIGS. 10–11 illustrate an exemplary embodiment of a shock absorber thatcan be integrated into a prosthetic structure.

FIG. 12A illustrates a top sectional view of a round post in a shockabsorber and FIG. 12B illustrates a side sectional view of a round postin a shock absorber.

FIG. 13 is a perspective view of a round post in a shock absorber.

FIGS. 14–27 illustrate exemplary embodiments of shock absorbers that canbe integrated into a prosthetic structure.

FIG. 28 is a top view of a transducer.

FIG. 29 is a bottom perspective view of a transducer.

FIG. 30 is a bottom view of a transducer.

FIG. 31 is a cross sectional view of a transducer.

FIG. 32 is a bottom view of a transducer with electrical leads.

FIG. 33 is a top view of a lower portion.

FIG. 34 is a cross sectional view of a lower portion.

FIG. 35 is a schematic diagram of a telemetry system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An exemplary prosthetic according to the present invention will now bedescribed. Generally, a prosthetic includes a component mounted to thefemur 2 and another component mounted to the tibia 4. Both femur 2 andtibia 4 are shown in dotted lines in FIG. 1. Although described withreference to a knee, those skilled in the art will recognizeapplicability of the present invention to other prosthetic structures.

FIG. 1 further illustrates assembly 10 in accordance with an exemplaryembodiment of the present invention. Assembly 10 includes femoralcomponent 12 mounted to the femur 2 and tibial component 14 mounted tothe tibia 4. Femoral component 12 includes flange 18 formed integrallywith two condyles 20. Femoral component 12 includes fixing posts oranchors 22 integrally formed on femoral component 12. Posts 22 are usedto fix the femoral component 12 to femur 2.

An outside surface 26 of flange 18 provides most of the bearing surfacefor a patella, not shown, which cooperates with femur 2 to protect thejoint. Condyles 20 are provided for replacing the condylar surfaces offemur 2 and include medial condylar surface 27 and lateral condylarsurface 28.

Tibial component 14 includes tibial inserts 30 and 32 and body portion34. Medial tibial insert 30 is adapted to engage medial condylar surface27, while lateral tibial insert 32 is adapted to engage lateral condylarsurface 28. The medial and lateral condylar surfaces 27 and 28 exertforce on medial tibial insert 30 and lateral tibial insert 32,respectively. Medial and lateral inserts 30 and 32 can be made frompolyethylene or any other suitable material.

During articulation of the knee joint, inserts 30 and 32 exert forces onbody portion 34. Body portion 34 includes medial plateau (or plate) 36,lateral plateau (or plate) 38 and lower plate 40. Support posts 42support the medial plateau 36 and lateral plateau 38. In one embodiment,body portion 34 serves as a transducer for measuring force articulatedwithin the knee joint. In this embodiment, strain gauges are mounteddirectly below support post 42 and sense strain therein. When installedas a replacement assembly for a natural human knee joint, assembly 10provides quantitative feedback in force load balance across thetibial-femoral joint. Tibial component 14 also includes a cover plate 43secured to lower plate 40 and fixing posts or anchors 44 to securetibial component 14 to tibia 4. In one embodiment, cover plate 43 iswelded to lower plate 40.

FIG. 2 illustrates a perspective view of tibial component 14 and tibia4. Tibia 4 has been resected (generally flat) to form a plateau 50.Cover plate 43 includes a major surface 52 adapted to engage plateau 50of tibia 4. In the embodiment illustrated, major surface 52 can includea plurality of apertures 54. The plurality of apertures 54 are connectedto passageways and are disposed substantially throughout major surface52 in order to enable growth of soft tissue, blood vessels, nerves, etc.from tibia 4 through the passageways in lower plate 40. A number ofdifferent configurations and quantity of apertures may be used. In oneembodiment, the plurality of apertures is at least ten apertures, andother embodiments include at least twenty-five and fifty apertures. Thegrowth is schematically indicated on plateau 50 at 56 as surfacevariations.

The design of body portion 34 provides several advantages. The variouscomponents of body portion 34 are elastic and act to dampen forcesplaced throughout the joint. Additionally, tibial component 14, sinceapertures in cover plate 43 provides an infusion of growth to thestructure of body portion 34, is more acceptable to the body.Additionally, only small fixing posts 44 are necessary in order tosecure tibial component 14 to tibia 4 prior to infusion. In prior artembodiments, a large center stem was necessary to secure tibialcomponents to respective tibias. The large center stem was particularlyinvasive to the body, which yielded an undesirable situation.Furthermore, soft tissue growth through lower plate 40 providesadditional damping for the knee joint. When pressure is placed on tibialcomponent 14, the soft tissue growth acts to dampen these forces, inparticular it minimizes shear forces on the knee joint. The soft tissuegrowth provides not only a more natural scenario, but can protect theknee joint from further failure. A further advantage in that weight oftibial component 14 is reduced by 35% over prior art components, in oneembodiment.

FIGS. 3–9 illustrate various views of portions of tibial component 14.FIGS. 5–6 illustrate medial plateau 36 and lateral plateau 38. Medialplateau 36 and lateral plateau 38 receive medial tibial insert 30 andlateral tibia insert 32, respectively. As appreciated by those skilledin the art, medial plateau 36 and lateral plateau 38 may be replacedwith a single plateau in one embodiment. Medial plateau 36 includes awall 58 defining a cavity 60 and lateral plateau 38 includes a wall 62defining a cavity 64. Walls 58 and 62 extend around the circumference ofmedial plateau 36 and lateral plateau 38 to receive the medial insert 30and tibial insert 32, respectively.

Medial plateau 36 and lateral plateau 38 are spaced apart to isolateforces from medial insert 30 and tibial insert 32, respectively. Asshown in FIG. 6, support posts 42 are cylindrical (round), particularlyat their coupling to the plateaus 36, 38 and plate 40, and support themedial plateau 36 and lateral plateau 38 to space the plates apart fromlower plate 40. The cylindrical posts 42 aid in extending the life ofthe prosthesis, but other curved shapes of posts so as to minimizestress concentration may be used. For each of the medial plate 36 andlateral plate 38, support posts 42 include three individual postsarranged in an isosceles triangle configuration. The configuration ofsupport posts 42 allows body portion 34 to closely resemble the naturalconfiguration of the mechanics of the knee during articulation andprovide added stability. The support posts 42 are also responsive toshear and torsion forces acting on the medial plateau 36 and lateralplateau 38 that are caused by various joint movements. Otherconfigurations of support posts may also be used.

Each of the bottom surfaces of medial plateau 36 and lateral plateau 38can include a plurality of grooves 66. In the illustrated embodiment,the plurality of grooves 66 extend in both a lateral direction andlongitudinal direction across the bottom surface of medial plateau 36and lateral plateau 38. The plurality of grooves 66 accommodates thegrowth of soft tissue, blood vessels, nerves, etc. from tibia 4 betweenmedial plateau 36 and lateral plateau 38 and lower plate 40. Althoughherein illustrated wherein the plurality of grooves 66 extend in lateraland longitudinal directions, various other groove configurations may beused in accordance with the present invention on the knee prosthesis orother joint prostheses.

FIGS. 7 and 8 illustrate views of lower plate 40. As illustrated in FIG.7, the upper surface of lower plate 40 can include a plurality ofgrooves 68 extending in lateral and longitudinal directions on the topsurface of lower plate 40. In one embodiment, the plurality of grooves68 of lower plate 40 are aligned with the plurality of grooves 66 ofmedial plateau 36 and lateral plateau 38. In addition to the pluralityof grooves, a plurality of apertures 70 extending normal to the topsurface of lower plate 40 are provided throughout lower plate 40. Boththe plurality of grooves 68 and plurality of apertures 70 are configuredto accommodate growth from tibia 4.

FIG. 8 illustrates a bottom perspective view of lower plate 40. Theplurality of apertures 70 are shown to extend through the lower plate40. A plurality of cavities 72 are further provided directly belowsupport post 42. Each of the cavities 72 serve as flexure members thatdampen forces placed on lower plate 40 by support posts 42. The flexuremembers 72 are deflected when forces are placed on support posts 42,which acts to dampen the forces and provide a more elastic prostheticstructure.

As illustrated in FIG. 9, major surface 52 of cover plate 43 includes aplurality of apertures 54 disposed substantially about the entire majorsurface. Cover plate 43 is secured to tibia 4, which provides additionalelasticity to the prosthesis with respect to compressive, shear andtorsion loads. The plurality of apertures 54 are substantiallyperpendicular to the major surface and extend through cover plate 43. Inthe embodiment illustrated, a plurality of apertures 56 are alsoprovided about a periphery of cover plate 43 on a surface substantiallyperpendicular to the major surface. The plurality of apertures 74extends radially outwardly but may be of any configuration. Theseplurality of apertures are connected to passageways that further allowgrowth of tissue from tibia 4 through cover plate 43. Various apertureconfigurations can be used.

The prosthetic structure described above is designed to provide anelastic structure that in one embodiment can also accommodate tissue,vessels and nerve growth into the structure. The structure acts todampen forces placed throughout the implant. Each of the separatecomponents are elastic, which contribute to the overall elasticity ofthe structure. The medial and lateral inserts together with the medialand lateral plateaus respond to static and dynamic loads independentlyand serve to dampen forces from the femur. The round posts supportingthe medial and lateral plateaus are associated with flexures thatdeflect under load, which ultimately diminishes vertical stiffness ofthe structure. Soft tissue, blood vessel and nerve growth infuse intoand around apertures in a cover plate and lower plate in order tofurther provide an elastic structure that will dampen forces placed onthe prosthetic implant. By dampening the forces placed on the prostheticstructure, loads, and particularly impact loads, placed on the kneejoint and other joints in the body are reduced.

An aspect of the present invention involves integrating a shock absorberinto a prosthetic structure such as described above. The embodimentsdescribed below can be integrated into the prosthetic structure indifferent ways including substituting the absorbers for elementsdescribed above and adding absorbers to the prosthetic structure. Theshock absorber acts to dampen forces placed upon the prostheticstructure to provide elasticity, which is helpful in maintainingdurability for prosthetic structures that experience numerous forcecycles throughout the life of the prosthetic structure.

FIGS. 10 and 11 illustrate one embodiment of a prosthetic shock absorberthat may be integrated into a prosthetic structure. Shock absorber 100includes a first plate 102 and a second plate 104 spaced apart fromfirst plate 102. A post 106 is provided between first plate 102 andsecond plate 104. In one embodiment, post 106 is substantially round,particularly at its connection to the first plate 102 and second plate104. The first plate 102, second plate 104 and post 106 form an integralstructure. Shock absorber 100 may be machined using a technique such aselectrical discharge machining (EDM) to form post 106. Andrew ToolCompany, located at 2405 Annapolis Lane, #266, Plymouth, Minn. 55441 canprovide such machining techniques. It is believed this process includesinitially machining square posts using the EDM wire process. Then, usingsmall or thin shaped electrodes, the corners are evaporated to form asubstantially round post. The electrode process may be a two-stepprocess, one side (e.g. semi-circular) of the post at a time. Themanufacturing of post 106 may cause fillets to be formed in post 106proximate first plate 102 and second plate 104. These fillets can act todistribute stress on the plates 102 and 104. Lower plate 104 alsoincludes a flexure 108 positioned to provide additional damping forshock absorber 100.

FIGS. 12A, 12B and 13 further illustrate post 106. The post 106 forms aradius that minimizes structural stress and optimizes electro-mechanicalperformance. As a result of the machining of post 106, a gap 110 isformed between first plate 102 and second plate 104. An exemplary gap110 can be in a range of about 0.04 to about 0.5 inches, althoughforming substantially round posts as described above in a prosthesiswith any gap and a flexure comprises an aspect of the present invention.Furthermore such a prosthesis with a gap less than 0.06 inches isparticularly advantageous. In another embodiment, a gap of 0.04 inchesis used. It should also be noted that the gap may vary depending uponthe type of material used and quality and/or accuracy of the machiningprocess. Some suitable materials include, but are not limited to,bio-compatible materials such as titanium and cobalt.

FIGS. 14 and 15 illustrate another embodiment of shock absorber 120 inaccordance with an aspect of the present invention. Absorber 120includes a first plate 122, a second plate 124, a post 126 and a flexure128. Post 126 and the posts described below in further embodiments aremade substantially round, at least at their connection to othercomponents such as plates and flexures, in a manner as described abovein the previous embodiments. In this embodiment, post 126 is hollow.Hollow post 126 forms a cavity 130. In a further embodiment, additionaldamping may be provided by filling cavity 130 with a substance. Examplesubstances include oil, gel and a liquid material. Cavity 130 may thenbe sealed to prevent the substance within cavity 130 from leaking.

FIGS. 16 and 17 illustrate yet another embodiment of a shock absorber140 including a first plate 142, a second plate 144 and a post 146. Inthis embodiment, first plate 142 includes a first flexure 148 and secondplate 144 includes a second flexure 150 to provide additional damping.

FIGS. 18 and 19 illustrate a shock absorber 160 similar to shockabsorber 140 illustrated in FIGS. 16 and 17. Shock absorber 160 includesfirst plate 162, second plate 164, post 166, first flexure 168 andsecond flexure 170. Additionally, post 166 is hollow and forms a cavity172. As discussed earlier, cavity 172 can be filled with a substancesuch as oil, gel and/or a liquid material and sealed to provideadditional damping.

The absorbers described above can also be combined in series or parallelto provide added damping. FIG. 20 illustrates a shock absorber 180 thatincludes absorbing elements in parallel. Shock absorber includes a firstplate 182 and a second plate 184. Substantially round posts 186, 188 and190 separate first plate 182 and second plate 184. Flexures associatedwith posts 186, 199 and 190 provided in second plate 184 includeflexures 192, 194 and 196, respectively.

FIG. 21 illustrates shock absorber 200, which is similar to shockabsorber 180 provided in FIG. 14. Absorber 200 includes first plate 202and second plate 204. A plurality of posts, including post 206, post 208and post 210 separate first plate 202 and second plate 204. Posts 206,208 and 210 also include associated flexures 212, 214 and 216,respectively. In this embodiment, posts 206, 208 and 210 includecavities 218, 220 and 222 that may be filled with a substance asdescribed above to provide additional damping.

FIG. 22 illustrates another shock absorber 230. Absorber 230 includes afirst plate 232 and a second plate 234. Absorber 230 also includessubstantially round posts 236, 238 and 240. Each of first plate 232 andsecond plate 234 include flexures associated with posts 236, 238 and240. First plate 232 includes flexures 242, 244 and 246 while secondplate 234 includes flexures 248, 250 and 252.

FIG. 23 illustrates another embodiment of a shock absorber 260. Absorber260 includes first plate 262, second plate 264 and substantially roundposts 266, 268 and 270. Plates 262 and 264 both include flexuresassociated with each of the posts 266, 268 and 270. First plate 262includes flexures 272, 274 and 276 and second plate 264 includesflexures 278, 280 and 282. In this embodiment, posts 266, 268 and 270are hollow to form cavities 284, 286 and 288, respectively. Cavities284, 286 and 288 may be filled with a substance to provide additionaldamping.

FIG. 24 illustrates another shock absorber 300 in accordance withanother embodiment of the present invention. Absorber 300 includesabsorbing elements 301, 303 and 305 arranged in series. Absorbingelement 301 includes a first plate 302 and a second plate 304 separatedby an associated post 306. Additionally, absorbing element 303 includesa first plate 306, a second plate 308 and a post 310. Absorbing element305 includes a first plate 311 and a second plate 312 separated by anassociated post 314. Additionally, plates 304, 308 and 312 includeflexures 316, 318 and 320, respectively. Absorbing elements 301, 303 and305 can be secured by welding or other suitable means.

FIG. 25 provides a similar embodiment to absorber 300 illustrated inFIG. 18. In this embodiment, absorber 330 includes absorbing elements331, 333 and 335. Absorbing element 331 includes plates 332 and 334.Element 333 includes plates 336 and 338 and element 335 includes plates340 and 342. Additionally, posts 344, 346 and 348 are provided inabsorbing elements 331, 333 and 335, respectively. Flexures 350, 352,and 354 are also provided in absorbing elements 331, 333 and 335,respectively. In this embodiment, posts 344, 346 and 348 are hollow andform cavities 356, 358 and 360, respectively, which can be filled with asuitable substance for additional damping.

FIGS. 26 and 27 illustrate similar absorbers to those illustrated inFIGS. 24 and 25, respectively. In the embodiment illustrated in FIG. 26,absorber 370 includes absorbing elements 371, 373 and 375. Absorbingelement 371 includes plates 372 and 374 spaced apart by substantiallyround post 376. Plates 372 and 374 include flexures 378 and 380,respectively. Absorbing elements 373 and 375 are similarly structured toabsorbing element 371. Element 373 includes plates 382 and 384, post 386and flexures 388 and 390, while element 375 includes plates 392 and 394,post 396 and flexures 398 and 399. In the embodiment illustrated in FIG.27, each of the posts are hollow and form a cavity that may be filledwith a suitable damping substance as described above.

As mentioned above, body portion 34 can be used as a force transducer tomeasure forces therein. FIGS. 28–32 illustrate an exemplary embodimentof a transducer according to the present invention. Transducer 400 issymmetrically u-shaped and constructed from suitable elastic materialthat is responsive to forces applied to medial and lateral plates 402and 404. Ultimately, transducer 400 is used to measure forces present onthe prosthetic components. The measurements can be used to properlyalign the components and analyze operation of the components.

FIG. 28 illustrates a top view of transducer 400. Medial plate 402 andlateral plate 404 are spaced apart to isolate forces placed on medialand tibial inserts that can be positioned on medial plate 402 andlateral plate 404. Both medial plate 402 and lateral plate 404 includecavities 410 and 412 that cam receive tibial inserts. Walls 414 and 416extend around the peripheral of plates 402 and 404 and define cavities410 and 412.

FIG. 29 illustrates a bottom perspective view of transducer 400 and FIG.30 illustrates a bottom plan view of transducer 400. FIG. 31 illustratesa sectional of transducer 400 taken along line 31-31 in FIG. 30. Asillustrated, lower plate 420 includes cavities 430, 432, 433, 434, 436and 438, which define flexures 440, 442, 443, 434, 436 and 438,respectively. In the embodiment illustrated, cavities 430, 432, 433,434, 436 and 438 are cylindrical with identical radii although otherconfigurations may be used.

Forces applied to medial and lateral plates 402 and 404 are localizedand directed through support posts 450 to a corresponding flexuremember. Sensors 460 measures deflection of flexures 440, 442, 443, 434,436 and 438 and can be resistive, capacitive, optical, etc. In theembodiment illustrated, a plurality of strain gauges are disposed ineach respective cavity on a surface of each respective flexure memberadjacent to support posts 450. Sensors 460 provide a quantitativeresponse to forces reacted between the medial and lateral plates 402,404 and lower plate 462, which correspond to forces carried by each ofthe condyles 20.

Flexures 440, 442, 443, 434, 436 and 438 allow forces to be measuredthroughout plate 402. Changes in forces can also be measured duringarticulation of the knee joint. Accordingly, an accurate replication offorces in a normal joint may be measured and analyzed. Incorrect loadingon an artificial joint can cause damage to connecting tissues such astendons and ligaments. Thus, by noticing incorrect loading, adjustmentsmay be made within the prostheses to insure proper performance.Apertures 464 in lower plate 462 are provided for fasteners (not shown)to secure transducer 400 to a lower portion. Other methods of securingtransducer 400 to a lower portion may be used, such as welding, bonding,etc.

FIG. 32 illustrates a bottom plan view of transducer 400 whereinelectrical leads from the sensors 460 are shown. Each of the medialplate 402 and the lateral plate 404 has an associated set of threesupport posts and three flexures arranged in an isosceles configuration.As illustrated, each set has a routing network 468 including channels470 that provide pathways for electrical leads from strain gaugeslocated in cavities 430, 432, 433, 434, 436, and 438. All electricalleads of the strain gauges are connected to a suitable connector orterminal strip 472 placed in cavity 474. Additional leads can connectterminal strip 472 to other circuitry that will acquire transducer data,process the data and transmit the data outside the body.

Sensors 460 are positioned in an “x” configuration, with eight gauges(four pairs) orthogonally arranged about a cylindrical post 450. Anynumber of sensing elements may be used. In the embodiment illustrated,the eight gauges in each flexure are connected in a conventionalWheatstone bridge configuration. The Wheatstone bridge configurationmeasures compressive loads on the flexures and minimizes cross talk fromshear and torsion loads. As appreciated by those skilled in the art,suitable sensors may be used to measure shear and torsion loads actingon the medial tray 402 and lateral tray 404.

Each flexure has an associated channel 470 that ultimately routeselectrical leads from sensors 460 to terminal strip 472. The channel 470may be offset with respect to their respective sensors 460, for exampleat an angle of 45° between adjacent sensors. For example, with referenceto cavity 430, the associated channel 470 is offset at an angle of 45°between the uppermost sensor and the rightmost sensor in cavity 430.Stated another way, the sensors 460 on each flexure are arranged inpairs with sensors from each pair positioned on opposite sides of thecorresponding support post to define two orthogonal reference linesillustrated, for example, at 473 and 475. It has been discovered that byorienting channels 470 to be oblique to the aligned pairs of sensors, oroblique to reference lines 473 and 475 improves transducer performance.In one embodiment, all channels 470 are oblique to all pairs of sensors.Minimizing the number of channels 470 exiting a flexure and offsettingthe channel with respect to associated sensors enhances accuracy and/orpredictability of signals obtained by sensors 460. As a result, signalsobtained include a more accurate representation of forces.

FIGS. 33–34 illustrate a lower portion 480, which can be secured totransducer 400. Lower portion 480 includes spikes 482 that secure thetransducer 400 to tibia 4 (FIG. 1). The spikes 482 are hollow to allowbone growth from tibia 4 through lower portion 480, which providesincreased stability and damping of forces throughout the knee joint.Additionally, a plurality of apertures 483 on major surface 484 andaround the circumference of lower portion 480 are connected topassageways and also allows soft tissue growth from tibia 4 throughlower portion 480. The apertures 482 are connected to passageways fortissue growth; however, it should be understood the location ofapertures 482 and orientation of passageways connected therebetween canvary as desired. Cone shaped portion 486 of lower portion 480 includes apocket 488 for storage of circuitry 490. Pocket 488 opens towardtransducer 400. Circuitry 490 is coupled to terminal strip 472 and isused to acquire, process and transmit transducer data.

As illustrated in FIG. 35, circuitry 490 can be a telemetry device thattransmits signals wirelessly to a receiver 492. Location of circuitry490 in pocket 488 of portion 480 provides an area for storage that issecure. More importantly though, the location below the transducer 400and thus on the tibia does not interfere with operation or stability ofthe knee joint. Receiver 492 can then transmit signals received from atelemetry device incorporating circuitry 490 to a computer 494 forfurther analysis.

As discussed above, the assembly accurately measures forces present onthe prosthesis in vivo or in vitro as the knee joint is articulatedthrough partial or complete range of movements. The resulting data iscollected and transmitted wirelessly for analysis to ensure proper forceload distribution across the load bearing surfaces of the knee jointprosthesis. With proper load distribution, the knee joint prosthesis isoptimally aligned thereby realizing increased prosthetic life.

Measurements obtained provide valuable information for research, surgeryand rehabilitation. For surgical applications, transducer 400 mayprovide data that aids in identifying locations where bone needs to beadded or removed to properly align the components of the prosthesis.Periodic monitoring of the assembly using telemetry allows forsystematic and timely diagnosis of potential problems within theassembly.

Although the present invention has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention.

1. A prosthesis comprising: a unitary body, comprising: a first plate; asecond plate spaced apart from the first plate and defining an outerperimeter; a substantially round post separating the first plate and thesecond plate and including a fillet at a connection point with the firstplate and a fillet at a connection point with the second plate; and aflexure in the second plate joined to the post and extending outwardlyfrom the post to completely surround the post and spaced apart from theouter perimeter of the second plate.
 2. The prosthesis of claim 1 andfurther comprising a plurality of substantially round posts separatingthe first plate and the second plate and wherein the second plateincludes a plurality of spaced apart flexures aligned with the pluralityof substantially round posts.
 3. The prosthesis of claim 2 wherein thefirst plate includes a plurality of spaced apart flexures aligned withthe plurality of substantially round posts.
 4. The prosthesis of claim 1wherein the round post is hollow to form a cavity.
 5. The prosthesis ofclaim 4 wherein the cavity is filled with a substance including at leastone of oil, gel and liquid material.
 6. The prosthesis of claim 1 andfurther comprising a third plate connected to the second plate, a fourthplate spaced apart from the third plate, a second substantially roundpost separating the third plate and the fourth plate and a secondflexure in the fourth plate proximate the second post.
 7. The prosthesisof claim 6 wherein the first-mentioned post and the second post share acommon axis.
 8. The prosthesis of claim 6 wherein the first-mentionedpost and the second post are hollow to form a first cavity and a secondcavity.
 9. The prosthesis of claim 8 wherein the first cavity and thesecond cavity are tilled with a substance including at least one of oil,gel and liquid material.
 10. The prosthesis of claim 6 wherein the firstplate and the third plate include flexures proximate the first-mentionedpost and the second post, respectively.
 11. The prosthesis of claim 1wherein the prosthesis is adapted to be placed in a knee joint.
 12. Theprosthesis of claim 1 and further comprising: a third plate spaced apartfrom the first plate to form a gap therebetween such that the thirdplate can move independently from the first plate; and a secondsubstantially round support post separating the third plate and thesecond plate.
 13. The prosthesis of claim 1 wherein a gap between thefirst plate and the second plate is less than or equal to 0.06 inches.14. A prosthesis comprising: a first plate; a second plate spaced apartfrom the first plate to form a gap that is less than or equal to 0.06inches; a substantially round post separating the first plate and thesecond plate and including a fillet at a connection point with the firstplate and a fillet at a connection point with the second plate, the postbeing hollow to form a cavity; and a flexure in the second platepositioned proximate the post.
 15. The prosthesis of claim 14 andfurther comprising a plurality of substantially round posts separatingthe first plate and the second plate and wherein the second plateincludes a plurality of spaced apart flexures aligned with the pluralityof substantially round posts.
 16. The prosthesis of claim 15 wherein thefirst plate includes a plurality of spaced apart flexures aligned withthe plurality of substantially round posts.
 17. The prosthesis of claim14 wherein the first plate, the second plate and the round post form anintegral structure.
 18. The prosthesis of claim 14 and furthercomprising a third plate connected to the second plate, a fourth platespaced apart from the third plate, a second substantially round postseparating the third plate and the fourth plate and a second flexure inthe fourth plate proximate the second post.
 19. The prosthesis of claim18 wherein the first-mentioned post and the second post share a commonaxis.
 20. A prosthesis comprising: a unitary body, comprising: a firstplate; a substantially round post connected to the first plate; a firstfillet extending from the post to the first plate; a second plateconnected to the post and spaced apart from the first plate by the postto form a gap between the first plate and second plate that is less thanor equal to 0.06 inches, the second plate including a flexure positionedtherein having reduced thickness relative to other portions of thesecond plate surrounding the flexure and completely surrounding thepost; and a second fillet extending from the post to the second plate.21. The prosthesis of claim 20 wherein the first plate includes aflexure positioned therein.